1 Field of the Invention
The present invention relates to a mold for curing an elastomeric article, such as a tire, and which mold has a vent passage communicating a cavity in the mold with the exterior of the mold. In particular, the present invention relates to a vent plug located in the vent passage for providing air bleed and restricting the amount of elastomeric material that may enter the vent passage.
2 Description of the Prior Art
It is known that during a tire molding operation, a plurality of relatively small projections are formed on ground engaging tread elements located about the outer circumference of a tire. These projections form by uncured elastomeric material flowing into vent passages in a mold. The projections then cure in the passages. The projections are generally considered visually unappealing and undesirable enough to require trimming away.
The projections have a further disadvantage because they can separate from the tire during stripping of the cured tire from the mold. A separated projection may remain in its respective passage, which renders the vent passage ineffective. A separated projection may also fall into the bottom of the mold. If the projection is not removed from the bottom of the mold, the projection can cause a visual defect in a tire that is formed subsequently in the mold.
The disadvantages associated with projections have been addressed mainly by inserting a vent plug into the vent passage. One such vent plug is disclosed in U.S. Pat. No. 4,436,497 and includes an opening extending longitudinally therethrough. The vent plug disclosed in U.S. Pat. No. 4,436,497 is inserted into a vent passage to reduce the effective cross-sectional area of the vent passage into which elastomeric material of an uncured tire may flow. The reduced cross-sectional area of the vent passage proportionately reduces the resistance to removal of the projection during stripping of the cured tire from the mold but also reduces the strength of projection. The end of the opening in the vent plug located adjacent the mold cavity is chamfered to thicken the base of the projection on the tire. The base of the projection is strengthened to reduce the propensity of the projection from breaking at the base. The chamfer increases resistance to tensile breakage but decreases resistance to shear failure. The thickened base, thus, may present a problem of the projection shearing off during stripping of the tire from the mold. Specifically, when the tread element is pulled or scraped across a rib on the mold, stresses may develop in the base of the projection which exceed the strength of the elastomeric material.
Other prior art systems for molding tires have centered on completely eliminating projections from a tire. For example, U.S. Pat. No. 4,740,145 discloses a vent plug made from a compressible material. The vent plug is porous and is located in a vent passage to project a predetermined distance into the mold cavity. When the uncured tire is pressed against the surface defining the cavity, the vent plug is compressed to substantially the same contour as the adjacent surface defining the cavity. The porosity of the vent plug permits venting of air through the vent passage but may be prone to clogging by the elastomeric material.